If you retreat a unit that your opponent targets with Hidden Blade, does the controller still draw 2 cards?

Ruling: No, the controller does not draw 2 cards. When Hidden Blade resolves, if its target is no longer a valid unit at a battlefield, the entire instruction is ignored because it cannot determine the controller of the target.

Sequence:

• Target legality is checked when Hidden Blade is added to the chain
• Target legality is checked again when Hidden Blade resolves
• If the target is not valid upon resolution (e.g., retreated), the instruction looking for "the controller of the unit at a battlefield" fails
• The spell still resolves (for effects that care about spell resolution), but the draw does not happen

Nuances:

• If the unit dies (like from Zhonya's or Sett), the draw still happens because the unit was still a legal target at a battlefield when Hidden Blade resolved
• If the unit is moved to another battlefield, the draw would still happen because the target remains legal
• Moving a unit away from the battlefield (Flash, Fade and Flourish, Retreat) makes the target invalid, preventing the draw